The services are started at the end of boot stage from: /DietPi/dietpi/postboot
Sadly you cannot natively disable/mask it, since it is already located at /etc/systemd/system/home-assistant.service, where masking would create a symlink to /dev/null usually to prevent the service from being started.
However you could move it to /lib/systemd/system/home-assistant.service, the place which is used by APT packages:mv /etc/systemd/system/home-assistant.service /lib/systemd/system/home-assistant.service
Then use:systemctl mask home-assistant ordietpi-services mask home-assistant